diff --git a/esphome/components/i2c/i2c.cpp b/esphome/components/i2c/i2c.cpp
index 82ab7bd09a..fdc9fd1ddf 100644
--- a/esphome/components/i2c/i2c.cpp
+++ b/esphome/components/i2c/i2c.cpp
@@ -7,6 +7,30 @@ namespace i2c {
 
 static const char *const TAG = "i2c";
 
+ErrorCode I2CDevice::read_register(uint8_t a_register, uint8_t *data, size_t len, bool stop) {
+  ErrorCode err = this->write(&a_register, 1, stop);
+  if (err != ERROR_OK)
+    return err;
+  return bus_->read(address_, data, len);
+}
+
+ErrorCode I2CDevice::write_register(uint8_t a_register, const uint8_t *data, size_t len, bool stop) {
+  WriteBuffer buffers[2];
+  buffers[0].data = &a_register;
+  buffers[0].len = 1;
+  buffers[1].data = data;
+  buffers[1].len = len;
+  return bus_->writev(address_, buffers, 2, stop);
+}
+
+bool I2CDevice::read_bytes_16(uint8_t a_register, uint16_t *data, uint8_t len) {
+  if (read_register(a_register, reinterpret_cast<uint8_t *>(data), len * 2) != ERROR_OK)
+    return false;
+  for (size_t i = 0; i < len; i++)
+    data[i] = i2ctohs(data[i]);
+  return true;
+}
+
 bool I2CDevice::write_bytes_16(uint8_t a_register, const uint16_t *data, uint8_t len) {
   // we have to copy in order to be able to change byte order
   std::unique_ptr<uint16_t[]> temp{new uint16_t[len]};
diff --git a/esphome/components/i2c/i2c.h b/esphome/components/i2c/i2c.h
index ffc0dadf81..780528a5c7 100644
--- a/esphome/components/i2c/i2c.h
+++ b/esphome/components/i2c/i2c.h
@@ -46,22 +46,10 @@ class I2CDevice {
   I2CRegister reg(uint8_t a_register) { return {this, a_register}; }
 
   ErrorCode read(uint8_t *data, size_t len) { return bus_->read(address_, data, len); }
-  ErrorCode read_register(uint8_t a_register, uint8_t *data, size_t len, bool stop = true) {
-    ErrorCode err = this->write(&a_register, 1, stop);
-    if (err != ERROR_OK)
-      return err;
-    return this->read(data, len);
-  }
+  ErrorCode read_register(uint8_t a_register, uint8_t *data, size_t len, bool stop = true);
 
   ErrorCode write(const uint8_t *data, uint8_t len, bool stop = true) { return bus_->write(address_, data, len, stop); }
-  ErrorCode write_register(uint8_t a_register, const uint8_t *data, size_t len, bool stop = true) {
-    WriteBuffer buffers[2];
-    buffers[0].data = &a_register;
-    buffers[0].len = 1;
-    buffers[1].data = data;
-    buffers[1].len = len;
-    return bus_->writev(address_, buffers, 2, stop);
-  }
+  ErrorCode write_register(uint8_t a_register, const uint8_t *data, size_t len, bool stop = true);
 
   // Compat APIs
 
@@ -85,13 +73,7 @@ class I2CDevice {
     return res;
   }
 
-  bool read_bytes_16(uint8_t a_register, uint16_t *data, uint8_t len) {
-    if (read_register(a_register, reinterpret_cast<uint8_t *>(data), len * 2) != ERROR_OK)
-      return false;
-    for (size_t i = 0; i < len; i++)
-      data[i] = i2ctohs(data[i]);
-    return true;
-  }
+  bool read_bytes_16(uint8_t a_register, uint16_t *data, uint8_t len);
 
   bool read_byte(uint8_t a_register, uint8_t *data, bool stop = true) {
     return read_register(a_register, data, 1, stop) == ERROR_OK;